Electrodynamic signal device



Dec. .2, 1930. c. R. HANNA 1,783,569

ELECTRODYNAMIC S IGNAL DEVI CE Original Filed Aug. 25, 1,925

INVENTOR Clinton R. Hanna ATT.ORNEY Patented Dec. 2, l93

UNITED STATES waste PATENT OFFICE CLINTON R. HANNA, OF EDGEWOOD, PENNSYLVANIA, ASSIGNOR T0 WESTINGHOUSE ELECTRIC & MANUFACTURING COMPANY, A CORPORATION OF PENNSYLVANIA ELECTRODYNAMIC SIGNAL DEVICE Original application filed August 25, 1925, Serial No. 52,268. Divided and this application filed July 11,

My invention relates to signal translating devices, more particularly to a microphone of the electrodynamic type and the circuit with which the microphone is associated.

It is an object of my invention to provide a combination of microphone and circuit of such character that the voltage delivered from the combination shall faithfully represent the sound.

In most devices for producing, from a sound, an electrical representation of that sound, distortion results because the response of the device to the sound is different at different frequencies. It is an object of my invention to provide a device in which said cause of distortion is largely avoided.

In a microphone of the electrodynamic type, that is, one in which the sound causes a conductor to move in a magnetic field, thus generating an electromotive force, the voltage delivered by the microphone is not in proportion to the sound, but varies with the frequency. Thus, if the sound exerts upon the microphone a force of constant amplitude but of increasing frequency, the voltage, instead of being constant, when the amplitude of the force exerted by the sound is con stant, will change as the frequency increases.

The change in the response of the instrument with change in frequency depends upon the damping, the nature of the forces tending to restore the vibrating system to normal position, and the inertia of said system. If the microphone is designed to have small damping and small restoring forces, the response will be inversely proportional to the frequency. It is an object of the invention to take advantage of this property of such microphone.

It is an object of my invention to devise a circuit having properties that will counteract the property of the microphone just described, thereby tending to keep the voltage impressed upon the line by the combination of microphone and circuit proportional to the force exerted by the sound and independent of the frequency.

In the practice of my invention, a circuit isused in which a reactor and a resistor are in series. When the frequency increases, the

Serial No. 291,895.

in the other direction, if it be acondenser.

It is an object of my invention to take advantageof this change with the frequency to eproduce a result independent of the frequ ncy It is a further object of my invention to provide a microphone of the electrodynamic type having very small internal reactance. By means of such a microphone, it is possible to design a combination of microphone and circuit in which the relation needed to produce an efiicient combination can be obtained without requiring any large inductance in the circuit external to the microphone. The small inductance present in such a circuit can be the primary of a transformer. This combination, consequently, enables me to use a transformer having a large turn-ratio and thus escapes the necessity that the microphone itself develop'a high Voltage.

In one form of my invention, the circuit to which the microphone is connected must have small reactance in proportion to its resistance.

It is an object of this invention to reduce the reactance of the microphone itself. By such reduction of the reactance, the resistance necessary to make the circuit predominantly resistive can be included in the microphone itself, thus avoiding the use of an external resistance.

It is a further object of this invention to provide a short-circuited secondary for the moving coil in the microphone, whereby the effective reactance of such coil is greatly diminished. One means for accomplishing this object is a plating of copper on surfaces adjacent to the air gap in the permanent magnetic circuit.

It has heretofore been proposed to provide a microphone free from abnormal responses to certain frequencies by utilizing the properties of a glow discharge in air. One disadvantage of such a microphone is that its properties change with atmospheric conditions.

Another disadvantage is that it requires a source of current'of very high potential whereby insulation difficulties are produced. My invention produces a microphone which is independent of atmospheric conditions and does not require that the potentials be high.

Other objects of the invention and structural details will he understood from the following description and the accompanying drawings, in which Figure 1 is a diagram of one form of the device,

Fig. 2 is a diagram of modification, and

Fig. 3 is a section through a portion of the microphone.

The microphone is provided with a dia phragm 1, preferably of the conical type, although the principle herein described is applicable to any microphone in which the principal reaction is that of inertia. The diaphragm is mechanically connected to a coil 2 which is reciprocated axially by the movement of the diaphragm. A magnet 3, either a permanent magnet or electromagnet as convenience dictates, provides an annular air gap in which the coil 2 moves.

The terminals of the coil are connected, as shown in Fig. 1, through a large resistor 5 to the primary 6 of a transformer. The resistance of the element 5 is so large in comparison with the inductance of the coil 6 that the impedance of the circuit as a whole does not change to any great degree with the frequency.

Preferably, the inductance of the coil 6 is equal to the effective inductance of the microphone. In order that the effective inductance of the microphone shall be as small as possible, the surfaces adjacent to the air gap are electro-plated, as shown at 7 and 8, with a material more conductive than the magnets. Each of the rings formed by the plating may be considered a short-circuited secondary, the coil 2 being regarded as a primary. The effect of the short-circuitcd secondary is to largely neutralize the inductance of the coil 2, producing, in effect, a voltage generator of greatly diminished internal reactance. I have found that this result may be obtained with a plating of copper only one mil in thickness. Obviously, either one of the two rings may be omitted, if desired. It IS equally obvious that a ring of some other form instead of a plating may be employed, when preferred.

The primary 6 of the transformer produces a voltage in the secondary 10 which is im pressed upon the grid 11 of an amplifier. The details of this amplifier are not illustrated since they are not related to this invention. The output circuit of this amplifier may be directly connected to the line or may boronuected through as many stages of amplification as necessary. At the outer end of the line I have shown a second amplifier 12 and a transformer 13 by means of which the signalling current is impressed upon the receiver 14. For obtaining the utmost advantage from the invention herein described, the

receiver 14 should also be free from abnormal responses at particular frequencies.

In the modification shown in Fig. 2, the microphone is connected directly to the primary6,without the interposition of a resistor. The secondary 10, instead of being connected directly to the grid 11, is connected through a condenser 15. In series with this condenser is a resistor 16, and the grid-cathode circuit of the tube is in sl nut to the resistor 16.

The capacity 0 the condenser 15 is so small that, even at the highest frequencies to which the microphone is subjected, the impedance of the condenser 15 is large, compared with the resistance of the element 16.

In the microphone, as illustrated in Fig. 3, the diaphragm 1 is supported by a means that offers practically no tendency to bring the diaphragm back to a mean position. The. support consists of a ring 17, preferably of very flexible material, such as kid leather. Its mechanical effect upon the coil 2 is merely that of a stop, preventing extreme excursion of the coil but leaving it otherwise free to reciprocate.

In the operation of the device, sound impressed upon the diaphragm 1 reciprocates the coil 2. Voltages are thus developed in the coil 2. The voltages being proportional to the velocity of the coil will be inversely proportional to the frequency of the sound. This relation between the frequency and the voltages holds for any microphone in which the moving parts are not subjected to damping or restoring forces, but present to the force exerted by the sound no resistance except that due to inertia.

In the circuit shown in Fig. 1, as the frequency of the sound increases, the voltage delivered by the microphone diminishes. The current through the circuit therefore also diminishes, because the impedance of the circuit is nearly constant. The current diminishing, the drop over the non-inductive element 5 diminishes.

The inductive element 6 offers a greater impedance at the higher frequency. This increase in impedance tends to counterbalance the effect of the decrease in current, whereby the drop over the inductor 8 is greater than the reduction in current would lead one to expect.

The impedance of the resistor 5 does not change with the frequency. The impedance of the reactor 6 does. The drop through the circuit is, therefore, differently distributed when the frequency increases, a larger portion of this drop being across the reactor 6.

This also-tends to counteract the effect of the diminution in current.

The two phenomena, each of which tends to counteract the effect of lessened current are together sufficient to completely counteract it, with the result that the potential across the primary 6 is unchanged by a change in the low frequency. This reactancc is the principal impedance in the circuit 1016. Diminution of this rcactance will, therefore, tend to increase the current and, consequently, will ofiset the diminution'in the voltage impressed upon the circuit. The current through the circuit 1016 is, therefore, substantially constant. Since the element 16 is n'oninductive, the constant current through it is accompanied by a constant voltage across it. The voltage between the grid and the cathode of the first amplifier tube is, therefore, independent of the frequency.

Since it is well known to those skilled in the art that, in general, a telephone transmitter of the electrodynamic type is capable of being used as a telephone receiver or loudspeaker, it is obvious that my invention is not limited to apparatus of the type hereinbefore referred to as microphones. If the moving coil of the microphone shown in. the drawings is supplied with current which is fluctuating, the reaction between the current and the fixed magnetic field will give rise to vibrations of the diaphragm corresponding to the current fluctuations, and the original sound will be faithfully reproduced. The reduction of the inductance of the moving coil by my invention assists very materially in increasing the range of sound-frequencies to which the loud speaker will efficiently re- .spond, and gives rise to a marked improvement in the quality of the sound-output therefrom.

l\'ly invention, therefore, is not to be restricted to the specific embodiment chosen for purposes of illuSt-rationfbut is to be limited only by the prior art or by the appended claims.

I claim as my invention:

1. In a microphone, a magnetic circuit having an air gap, a coil mounted to move in said air gap, means for moving said coil in accordance with the sound and a conductive plating constituting a short-circuited secondary for said coil.

2. In a microphone, a magnetic circuit having an annular air gap, a coil mounted to move in said air gap, and means for moving said coil in accordance with the sound, one surface adjacent to said air gap being coated with conductive material.

3. In a microphone, a magnetic circuit having an annular air gap, a coil mounted to move in said air gap, and means for moving said coil in accordance with the sound, one surface adjacent to said air gap being copper plated.

4. In a microphone, a magnetic circuit having an annular air gap, a coil mounted to move in said air p, and means for moving said coil in acco ance with the sound, the surfaces adjacent to said air gap being copper plated.

5. In a sound-translating device, a magnetic circuit comprising elements which constitute an air-gap, a coil mounted for movement in said air-gap, and means, comprising a conductive coating on certain of said elements, for reducing the inductance of said coil.

6. In a sound-translating device, a magnetic circuit having an air-gap, a coil mounted for movement in said air-gap, a diaphragm attached to and movable with said coil, and means, comprising a conductive coating on certain of the elements constituting said air-gap, for reducing the inductance of said coil.

7. In a sound-translating device, a magnetic circuit comprising an air-gap, a coil mounted for movement in said air-gap, a dia phragm attached to and movable with said coil, and means, comprising a conductive coating on certain of the elements constituting said air-gap, for reducing the inductance of said coil.

8. In a sound translating device, a magnetic circuit comprising an air-gap, a coil mounted for movement in said air-gap, and means, comprising a conductive coating of non-magnetic material on certain of the elements of said air-gap, for reducing the inductance of said .coil. 7

9. In a sound translating device, a magnetic circuit comprising an air-gap, a coil mounted for movement in said air-gap, and means, comprising a conductive coating of non-magnetic material on certain of the elements of said air-gap adjacent said coil, for constituting a short-circuited secondary for said coil, whereby the apparent inductance of'the coil is reduced.

7 10. In asound-translating device, a magnetic circuit comprising an annular air-gap, a relatively large conical diaphragm, a. coil attached to the apex of said diaphragm and mounted for movement in said air-gap, and means, comprising a conductive coating of non-magnetic material on certain of the elements of said air-gap, for reducing the apparent inductance of the coil, whereby the impedance of said coils to currents at high frequencies is lessened.

11. A speech current apparatus including a pair of magnetic pole pieces, layers of conductive material arranged to cover the active its areas o f hoth of said pole pieces, a diaphragm, and a coil mounted on said diaphragm and arranged to move between said layers of conductive material.

12. A speech current apparatus including a pair of magnetic pole pieces, layers of nonmagnetic conductive material arranged to cover the active areas of both of said pole pieces, a diaphragm, and a coil mounted on said diaphragm and arranged to move between said layers of nonmagnetic conductive material.

13. A speech current apparatus including a pair of magnetic pole pieces, layers of copper arranged to cover the active areas of both of said pole pieces, a diaphragm, and a coil mounted on said diaphragm and arranged to move between said layers of copper.

14. A speech current apparatus including a pair of magnetic pole pieces, the active area of one of said pole pieces being electroplated Wherehy it is covered with a thin layer of conductive material, a diaphragm, and a coil mounted on said diaphragm and arranged to move between said pole pieces.

In testimony whereof, I have hereunto subscribed my name this 6th day of July, 1928.

CLINTON R. HANNA. 

